1. Field Of The Invention
The invention is in the field of coupling optical guides. More particularly, the invention relates to a method of positioning and fixing optical fibres in a row of optical fibres such as is used in a coupling device to couple a number of optical fibres to a corresponding number of optical channels, and to a coupling device provided with such a row of fibres.
2. Prior Art
In optical networks, in which the transmission of light signals takes place via glass fibre links, integrated optical components are being used evermore frequently to generate, process, switch, etc. the light signals in such networks. In this connection, it is necessay for there to be lightguide channels, which can be coupled to the glass fibres, in the optical components for coupling the light signals in and out. For this purpose, the lightguide channels in the optical components are made as equal as possible geometrically and optically, but at least in terms of modal field diameter, to the glass fibres to be coupled thereto. A link having only low losses is possible without the need to use coupling lenses if each pair of optical waveguides to be coupled, in this case a lightguide channel and a glass fibre, are glued to each other in mutual alignment by means of well polished end faces which are usually chosen at right angles. In such a coupling, three mismatches may give rise to undesirable coupling losses, namely a transverse displacement, a longitudinal displacement and an angular rotation of the optical waveguides with respect to each other. On a chip, integrated optical components are usually so designed that a number of optical waveguide channels to be coupled emerge at the edge of the chip into a common coupling face in which the said end faces of the channels are situated. In the vicinity of said coupling face, said channels form a planar row of parallel optical waveguides placed close together. This makes it difficult to couple every channel in the coupling face separately to a glass fibre with sufficient accuracy. In order to avoid such separate coupling, use is in fact made of a coupling device which comprises a substrate on which a number of optical fibres to be coupled are situated with one end positioned and fixed in such a way that the cores of the fibres can be integrally aligned in the coupling face with the waveguide channels on the chip, while the fibres have sufficient room at their other end for separate coupling to other optical fibres. Such coupling devices are known, for example, from references [1] to [6] referred to below under C. In said known coupling devices, referred to as glass fibre arrays, the fibre ends, preferably stripped of their primary coating, of a number of parallel optical fibres lie fixed and positioned in a housing at a mutually very accurate centre-to-centre distance. In that case, the housing is formed by two parallel clamping plates, a base plate and a cover plate, between which the fibres are clamped. Two techniques are known for the accurate mutual positioning of the fibres. A first technique, as disclosed by reference [1], makes use of accurately positioned grooves provided in the base plate, in which grooves the fibres are laid during manufacture and then fixed by gluing the cover plate thereon. The use of grooves, which have to be produced beforehand, for positioning the optical fibres and the placing of each fibre separately in such a groove make this first technique for manufacturing said coupling devices fairly laborious and expensive, certainly in the case of large numbers of fibres, for example 10 or more. In addition, the coupling devices obtained therewith are not very compact. However, a greater compactness, accompanied by a less laborious manufacture, can be achieved if, for the accurate mutual positioning of the fibres, use is made of the fact that the current generation of optical fibres is manufactured with an accuracy such that the geometrical parameters thereof, such as concentricity, ellipticity and external diameter, are constant within very narrow tolerances. According to a second known technique, the fibres in that case lie very contiguously alongside one another in their housing, with their common lines of contact and the centre lines of the fibres essentially in a flat plane. Three of the references mentioned, namely [2], [3] and [4], disclose designs in which the fibres are clamped between two clamping plates. However, the two clamping plates are now each provided, diametrically with respect to one another, with a stepped layer having a height of approximately the diameter of a fibre so that there is, between the clamping plates, an opening having a rectangular cross section, in which the row of fibres is placed. During the manufacture of the fibre array, the fibres are first placed on the base plate alongside one another with a generous intervening spacing and with the stepped layer on one side. Then the cover plate with its stepped layer is placed on the fibres on their other side an pressed down until a mutual spacing from the base plate somewhat larger than the fibre diameter is reached. Then the base plate and cover plate are successively pushed home horizontally and vertically, as disclosed by reference [2], or, alternatively, in a diagonal direction, as disclosed by reference [3], until the fibres are, as it were, swept together and lie closely adjacent between the two stepped layers. Reference [5] discloses a variant of this in which the cover plate consists of two parts, each provided with a stepped layer, with which the fibres are swept together on the base plate until they lie in the desired closely adjacent position. Reference [6] furthermore also discloses sucking the fibres on via a suction opening in the base plate until they are in the desired position and then pressing them down in said position from above with a cover plate, during which process the space between the base plate and the cover plate on either side of the fibres is filled up with layers comparable to the stepped layers mentioned.
After achieving accurate positioning in this way, the fibres are fixed by further gluing of the parts of the housing and filling up the spaces between the housing parts and the fibres with suitable material, such as casting resin. Then an assembly obtained in this way is sawn through the two clamping plates along a cut perpendicular to the axis of the fibres, and one or each of the two pieces obtained in this process is rendered suitable by polishing for a direct contact coupling to a coupling face of an optical chip.
The manufacture of fibre arrays by the second techique as disclosed by references [2] to [5] inclusive has two problems. First of all, the vertical and horizontal forces which have to be exerted on the clamping plates in order to place the fibres in the closely adjacent position are difficult to adjust independently of one another, with the possible consequence that the fibres start to give way. Problems may also arise in this connection with heating. A second problem is that, if a large number of fibres are laid in parallel alongside one another in a preliminary position, crossovers occur very readily under these circumstances unless a relatively large intervening spacing is chosen at the same time, with the result that the clamping plates have to have a certain excess length solely for the purpose of manufacture. Although the manufacture of fibre arrays by the second technique as disclosed by reference [6] does not have the first problem mentioned, the measures necessary for applying suction forces complicate the manufacture.